Operational amplifiers (“opamps”) are basic blocks utilized in a wide range of electronic circuits. In addition to amplification and buffering, opamps are typically used to implement functions such as summing, integration, multiplication, and differentiation. Typical opamp applications include comparators, oscillators, filters, sample and hold circuits, and instrumentation amplifiers.
Multiple-stage operational amplifiers (opamps) typically include a cascade of one or more gain stages and an output driver stage. The output stage is, for example, a Class AB amplifier that provides load drive capability. To achieve an overall high open loop gain (e.g. greater than 150 dB), a multiple-stage opamp normally requires three or more gain stages.
Opamps are often subject to an inherent input-referred offset voltage. Generally, when the voltages at the differential inputs of the opamp are equal, the output voltage should theoretically be at the mid-supply voltage. In actual applications, a slight offset in the output voltage from the mid-supply voltage occurs when the input voltages are equal. For some opamp applications, input-referred offset is not acceptable, and therefore various techniques have been developed for minimizing input-referred offset.
One technique for minimizing input-referred offset is to match the input differential transistor pair and the load transistor pair of one or more of the internal stages, and typically the first stage. Another technique is to increase the sizes of the input transistor pair. However, even when these techniques are utilized, some finite input referred offset still remains.
A third technique for addressing the problem of input-referred offset is chopper stabilization. In chopper stabilization, the signal of interest at the input of one or more stages of an opamp is modulated or “chopped” at a high frequency. Typically, the chopping frequency is selected to be at least twice the frequency of the band of the signal of interest to avoid aliasing. At the output of the chopper-stabilized stage, the signal of interest is demodulated back into the original signal band by a second chopping operation. This second chopping modulates any inherent offset and/or flicker (1/f) noise out of the frequency band of the signal of interest. While often utilized, chopper stabilization nevertheless generates artifacts in the opamp output, which are often also not acceptable.
For high performance applications, such as instrumentation amplifiers, improved techniques for addressing the problem of input-referred voltage offset in opamps are required. In particular, these techniques should allow for the implementation of chopper-stabilization without the introduction of an excessive number of artifacts in the opamp output. These techniques should not unduly complicate the overall opamp design, significantly increase the power consumption of the overall device, or require a substantial amount of additional chip area to fabricate.